This invention relates generally to apparatus and methods for measuring strain. More particularly, this invention relates to measuring strain along a curved surface. Still more particularly, this invention relates to a strain sensor having a flexible fiber optic sensing element.
Fiber optic sensors have a wide variety of applications for sensing parameters such as temperature, pressure, strain, acoustic waves, electromagnetic waves and rotation. Strain in an optical fiber produces an optical path length change by physically changing the length of the fiber and by changing its index of refraction by means of the photoelastic effect. A number of parameters may be measured when the optical fiber is mounted in a transducer that produces a strain in the fiber when the parameter of interest changes.
The most sensitive fiber optic sensors detect changes in a parameter of interest by monitoring interference between two signals. One or both of the optical signals are exposed to the parameter. The signals have different optical paths and experience a relative phase change as the parameter changes. Commonly used interferometric sensors include Mach-Zehnder, Michelson, Fabry-Perot, ring resonator, polarimetric and two-mode fiber interferometers. Most of these sensors have two separate fibers that form sensing and reference arms. A length of a single fiber may function as an interferometer if it guides two modes that exhibit different responses to changes in the parameter being measured.